CWE 404

The program does not release or incorrectly releases a resource before it is made available for re-use.

Extended Description

When a resource is created or allocated, the developer is responsible for properly releasing the resource as well as accounting for all potential paths of expiration or invalidation, such as a set period of time or revocation.

Time of Introduction

Architecture and Design

Implementation

Applicable Platforms

Languages

All

Common Consequences

Scope

Effect

Availability

Most unreleased resource issues result in general software reliability problems, but if an attacker can intentionally trigger a resource leak, the attacker might be able to launch a denial of service attack by depleting the resource pool.

Confidentiality

When a resource containing sensitive information is not correctly shutdown, it may expose the sensitive data in a subsequent allocation.

Likelihood of Exploit

Low to Medium

Demonstrative Examples

Example 1

The following method never closes the file handle it opens. The Finalize() method for StreamReader eventually calls Close(), but there is no guarantee as to how long it will take before the Finalize() method is invoked. In fact, there is no guarantee that Finalize() will ever be invoked. In a busy environment, this can result in the VM using up all of its available file handles.

(Bad Code)

Example Language: Java

private void processFile(string fName) {

StreamWriter sw = new

StreamWriter(fName);

string line;

while ((line = sr.ReadLine()) != null)

processLine(line);

}

Example 2

If an exception occurs after establishing the database connection and before the same connection closes, the pool of database connections may become exhausted. If the number of available connections is exceeded, other users cannot access this resource, effectively denying access to the application. Using the following database connection pattern will ensure that all opened connections are closed. The con.close() call should be the first executable statement in the finally block.

(Bad Code)

Example Language: Java

try {

Connection con = DriverManager.getConnection(some_connection_string)

}

catch ( Exception e ) {

log( e )

}

finally {

con.close()

}

Example 3

Under normal conditions the following C# code executes a database query, processes the results returned by the database, and closes the allocated SqlConnection object. But if an exception occurs while executing the SQL or processing the results, the SqlConnection object is not closed. If this happens often enough, the database will run out of available cursors and not be able to execute any more SQL queries.

(Bad Code)

Example Language: C#

...

SqlConnection conn = new SqlConnection(connString);

SqlCommand cmd = new SqlCommand(queryString);

cmd.Connection = conn;

conn.Open();

SqlDataReader rdr = cmd.ExecuteReader();

HarvestResults(rdr);

conn.Connection.Close();

...

Example 4

The following C function does not close the file handle it opens if an error occurs. If the process is long-lived, the process can run out of file handles.

(Bad Code)

Example Language: C

int decodeFile(char* fName) {

char buf[BUF_SZ];

FILE* f = fopen(fName, "r");

if (!f) {

printf("cannot open %s\n", fName);

return DECODE_FAIL;

}

else {

while (fgets(buf, BUF_SZ, f)) {

if (!checkChecksum(buf)) {

return DECODE_FAIL;

}

else {

decodeBlock(buf);

}

}

}

fclose(f);

return DECODE_SUCCESS;

}

Example 5

In this example, the program fails to use matching functions such as malloc/free, new/delete, and new[]/delete[] to allocate/deallocate the resource.

(Bad Code)

Example Language: C++

class A {

void foo();

};

void A::foo(){

int *ptr;

ptr = (int*)malloc(sizeof(int));

delete ptr;

}

Example 6

In this example, the program calls the delete[] function on non-heap memory.

Use a language with features that can automatically mitigate or eliminate resource-shutdown weaknesses.

For example, languages such as Java, Ruby, and Lisp perform automatic garbage collection that releases memory for objects that have been deallocated.

Phase: Implementation

It is good practice to be responsible for freeing all resources you allocate and to be consistent with how and where you free memory in a function. If you allocate memory that you intend to free upon completion of the function, you must be sure to free the memory at all exit points for that function including error conditions.

Phase: Implementation

Memory should be allocated/freed using matching functions such as malloc/free, new/delete, and new[]/delete[].

Phase: Implementation

When releasing a complex object or structure, ensure that you properly dispose of all of its member components, not just the object itself.

Phase: Testing

Use dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Phase: Testing

Stress-test the software by calling it simultaneously from a large number of threads or processes, and look for evidence of any unexpected behavior. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Phase: Testing

Identify error conditions that are not likely to occur during normal usage and trigger them. For example, run the program under low memory conditions, run with insufficient privileges or permissions, interrupt a transaction before it is completed, or disable connectivity to basic network services such as DNS. Monitor the software for any unexpected behavior. If you trigger an unhandled exception or similar error that was discovered and handled by the application's environment, it may still indicate unexpected conditions that were not handled by the application itself.

Weakness Ordinalities

Ordinality

Description

Primary

Failing to properly release or shutdown resources can be primary to resource exhaustion, performance, and information confidentiality problems to name a few.

Resultant

Failing to properly release or shutdown resources can be resultant from improper error handling or insufficient resource tracking.